Photometer including variable amplification and dark current compensation

ABSTRACT

Apparatus for measuring the illumination from a source of light is disclosed in accordance with the teachings of the present invention wherein a photodiode exhibiting a substantially uniform spectral sensitivity within a defined range generates a current proportional to the intensity of illumination thereon. Operational amplifier means including a first operational amplifier having a constant closed loop amplification factor connected in series relationship with a second operational amplifier exhibiting a selectively variable closed loop amplification factor amplifies the current generated by said photodiode to provide a voltage that is proportional to the intensity of the illumination of said photodiode. Indicating means may be coupled to said operational amplifier means to provide an indication of the intensity of said illumination.

United States Hostetter Patent [191 ,PHOTOMETER INCLUDING VARIABLEJoseph A. Hostetter, Fairport, NY.

[75] lnventor:

[73] Assignee: Xerox Corporation, Stamford, v Conn.

[22] Filed: Dec. 22, 1971 [21] Appl. No.1 210,966

[52] US. Cl 356/226, 250/214, 330/86 [51] Int. Cl. G0lj 1/44 [58] Fieldof Search 356/226; 250/214, 206; 324/123; 328/142; 330/110, 86

[56] References Cited UNITED STATES PATENTS 3,614,444 10/1971 Nirschi330/110 3,509,474 4/1970 Arnold et al 328/142 3,514,209 5/1970 McGhee eta1... 250/214 3,664,752 5/1972 Hermieu 356/224 3,382,451 5/1968 Giletteet al. 324/123 3,667,036 5/1972 Seachman 324/123 r LQ -v\7 t"\ 15 ,2 \l28 F z e Feb. 5, 1974 [57] ABSTRACT Apparatus for measuring theillumination from a source of light is disclosed in accordance with theteachings of the present invention wherein a photodiode exhibiting asubstantially uniform spectral sensitivity within a defined rangegenerates a current proportional to the intensity of illuminationthereon. Operational amplifier means including a first operationalamplifier having a constant closed loop amplification factor connectedin series relationship with a second operational amplifier exhibiting aselectively variable closed loop amplification factor amplifies thecurrent generated by said photodiode to provide a voltage that isproportional to the intensity of the illumination of said photodiode.Indicating means may be coupled to said operational amplifier means toprovide an indication of the intensity of said illumination.

6 Claims, 2 Drawing Figures PI-IOTOMETER INCLUDING VARIABLE IAMPLIFICATION AND DARK CURRENT v COMPENSATION This invention relates toapparatus for measuring the illumination from a source of light and moreparticularly to a light detector exhibiting linear stable operation inresponse to radiant flux of different wavelengths.

In many applications it is necessary to determine the intensity of lightemitted by a source of light and incident on a surface. One suchapplication resides in the manufacture and testing ofelectrophotographic reproducing devices. In these devices aphotoconductive surface is exposed to a light image such that latentimages are formed thereon. The latent images are then developed andtransferred to a support surface. Further description of conventionalelectrophotographic devices may be found in US. Pat. No. 2,297,691 whichissued to CF. Carlson. v

The light image to which the photoconductive surface is exposed isproduced by modulating the light emitted by a light source withcharacter patterns such as graphic information. For the successfuloperation of such electrophotographic devices, it is necessary that theintensity of unmodulated light transmitted to the photoconductivesurface admit of a specified value. Accordingly during the course ofmanufacturing these devices-the proper operation of the light source isdetermined by measuring the intensity of the illumination from saidlight source at a surface corresponding to the photoconductive surface.

The prior art has developed many devices for measurin g the intensity ofillumination at a surface. Most of these devices utilize phototubeswhich generate currents when radiant flux is incident on the surfacethereof. An attendant disadvantage therewith is that the magnitude ofthe current generated by a conventional phototube is dependent upon thewavelength of incident light. In fact, most phototubes are characterizedby a spectral sensitivity in the infrared region. However, mostphotoconductive materials conventionally utilized in electrophotographicdevices respond to visible light and to light in the ultraviolet regionof the spectrum. Therefore it is desirable to provide a light detectorhaving a corresponding spectral sensitivity.

Another disadvantage of light measuring devices utilizing prior artphototubes is that the operation of such devices is stable only duringshort periods of time. Consequently these light measuring devices mustbe frequently calibrated and readjusted to assure desired performance.

Still another disadvantage is that light detectors heretofore developedby the prior art exhibit non-uniform operating characteristics. In otherwords, the operation of one such light detector may not necessarilycorrespond to the operation of a second light detector comprised ofsimilar components. This factor tends to defeat the desired objective ofstandardization in measuring the intensity of illumination, resulting intest data that are not easily correlated.

Therefore it is an object of the present invention to provide aninexpensive light detector that exhibits stable operation over longperiods of time.

It is a further object of this invention to provide a light detectorhaving a substantially linear operating characteristic such that each ofa plurality of like light detectors admits of identical operation.

It is still another object of this invention to provide a light detectorfor measuring the intensity of relatively low level illumination.

A still further object of this invention is to provide apparatus-formeasuring a broad range of intensity of radiant flux.

Another object of the present invention is to provide a light detectoremploying a photosensitive silicon device wherein the reverse saturationcurrents thermally generated by said device are minimized.

Various other objects and advantages of the invention will become clearfrom the following detailed description of exemplary embodiments thereofand the novel features will be particularly pointed out in connectionwith the appended claims.

In accordance with this invention, a light meter for measuring radiantflux of various wavelengths is provided wherein a photosensitive silicondevice exhibiting a substantially uniform spectral sensitivitycharacteristic to radiant flux generates a current proportional to theintensity of the radiant flux received thereby which current isamplified by operational amplifier means; said operational amplifiermeans includes first and second series connected operational amplifiershaving constant and selectively variable closed loop amplificationfactors, respectively; and the output of said operational amplifiermeans is applied to indicating means for providing an indication of theintensity of said received radiant flux.

The invention will be more clearly understood by reference to thefollowing detailed description of exemplary embodiments thereof inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a first embodiment of a light meter inaccordance with the present invention; and

FIG. 2 is a schematic diagram of a second embodiment of a light meter inaccordance with this invention.

Referring now to the drawings wherein like reference numerals are usedthroughout, and in particular to FIG. 1, there is illustrated aschematic diagram of a light meter in accordance with the presentinvention comprising a photodiode l1, operational amplifier I2,operational amplifier I6 and indicating device 35. The photodiode 11 maycomprise a conventional silicon photodiode having a substantiallyuniform spectral sensitivity to radiant energy admitting of wavelengthsin the range 400 to l,000 millimicrons. A typical photodiode isidentified as Model No. SGD444 and manufactured by Edgerton Germeshausenand Grier Inc. As is understood by those skilled in the art, photodiode11 is adapted to generate a current proportional to the intensity ofradiant flux incident thereon. Accordingly the photodiode 11 may bedisposed in a plane at a desired distance from a source of light 10whereat the intensity of the illumination from the source of light 10 isto be measured. The cathode of photodiode I1 is coupled to a source ofbiasing potential not shown, and the anode of the photodiode 11 iscoupled to an input terminal of operational amplifier 12 by an inputresistance means 13. If desired, a silicon phototransistor whichexhibits operating characteristics that correspond to those of siliconphotodiodes may be utilized in place of the photodiode 1 1. Accordinglythe description set forth hereinbelow with reference to a photodiode isequally applicable to a phototransistor.

Operational amplifier 12 may comprise a conventional integrated circuitoperational amplifier such as Motorola Model No. MC l 5396 or theequivalent. One skilled in the art will recognize that operationalamplifier 12 exhibits high input impedance and a high open loopamplification factor. The operationalamplifier 12 may comprise theaforementioned integrated circuit or may be constructed of conventionaldiscrete components. In addition the operational amplifier may includean inverting input terminal and a noninverting input terminal. In theconfiguration illustrated in FIG. 1, input resistance means 13 iscoupled to the inverting input terminal of operational amplifier l2, andresistance means 15 is coupled to the noninverting input terminal ofoperational amplifier 12. It is of course recognized that the foregoinginput terminals of operational amplifier 12 may be readily interchanged.The resistance means 15 is coupled to ground potential and is aconventional drift stabilizing resistor normally utilized to insurestable operation of an operational amplifier, but may be omitted if sodesired. The operational amplifier 12 further includes feedbackresistance means 14 whereby the output terminal of said operationalamplifier is coupled to the inverting input termi nal thereof. Hence oneskilled in the art will recognize that the operational amplifier 12functions as a wellknown inverting amplifier, and if the resistances ofresistance means 13 and '14 respectively are fixed, the closed loopamplification factor is determined by the value of the resistance ofresistance means 14 divided by the value of the resistance ofresistance-means 13. Although the input terminals of the operationalamplifiers illustrated herein are designated inverting and non-invertinginput terminals, such terminology is adopted merely for convenience.Clearly, either input terminal may be an inverting terminal.Furthermore, operational amplifiers having but a single input terminalmay be utilized.

The output terminal of operational amplifier 12 is coupled to aninverting input terminal of operational amplifier 16 via resistancemeans17. The latter operational amplifier may be similar to aforedescribedoperational amplifier l2 and need not be explained in further detailherein. A non-inverting input terminal of operational amplifier 16 iscoupled to a reference potential such as ground potential by resistancemeans 27 which comprises a drift stabilizing resistor. The noninvertinginput terminal of the operational amplifier 16 is further coupled to anadjustable contact 30 of potentiometer 29 via resistance means 28 for apurpose soon to be described. The potentiometer 29 is connected inseries relationship with terminals 31 and 32 which are adapted to besupplied with biasing potentials. The output terminal of operationalamplifier 16 is adapted to be selectively coupled to the inverting inputterminal thereof via common connected resistance means 20,22,24 and 26.Each of said resistance means has a first terminal thereof connected incommon relationship to the output terminal of operational amplifier l6and a second terminal thereof connected to stationary switch contacts19, 21, 23 and 25 respectively. A moveable armature 18 is adapted toconnect the inverting input terminal of operational amplifier 16 to oneof said switch contacts 19 25. The moveable armature l8 and stationaryswitch contacts 19 25 preferably comprise a conventional rotary switch.However it is to be understood that other conventional switch means suchas relay switch means, transistor switch means or the like may beutilized to selectively couple the output terminal of operationalamplifier 16 to the inverting input terminal thereof via one of theresistance means 20 26. it is now apparent that operational amplifier 16is adapted to assume the configuration of an inverting amplifier havinga closed loop amplification factor dependent upon the value of each ofthe selectively connectable resistance means 20 26. If the value of theresistance of each of the resistance means 20 26 is consecutivelyincreased by an order of 10, it is understood that the amplificationfactor, or gain of. the operational amplifier 16 may be selectively be0, 20, 40, or decibels in accordance with the positioning of moveablearmature 18 with respect to the stationary switch contacts 19 25. Ifdesired, each of resistance means 20 26 may have a first terminalthereof connected in common relationship to the inverting input terminalof operational amplifier l6 and moveable armature 18 may be connected tothe output terminal of operational amplifier 16. Further, the inputterminals of operational amplifier 16 may be readily interchanged.

The output terminal of operational amplifier 16 is additionally coupledto indicating means 35 via adjustable contact 34 of potentiometer 33.The indicating means 35 is adapted to display the voltage produced atsaid output terminal and therefore may comprise a conventional voltmetersuch as a digital voltmeter, a vacuum tube voltmeter, cathode my deviceor the like. lt is preferable to employ a digital voltmeter such as aFair child Digital voltmeter Model 7050. The potentiometer 33 admits ofa resistance that is much less than the input impedance of theindicating means and is adjustable so that indicating means 35 may becalibrated in units of radiant flux density such as microwatts persquare centimeter. Accordingly the voltage produced at the outputterminal of operational amplifier 16 and applied to indicating means 35may be utilized as a direct measure of the illumination of thephotodiode 11.

The operation of the light meter schematically illustrated in FIG. 1will now be described. The radiant energy transmitted by the source oflight 10 is received as radiant flux on the surface of the photodiode 11opposite thereto. The source of light 10 may comprise a conventionalincandescent lamp, a fluorescent lamp or a source of monochromaticlight. Inasmuch as photodiode ll exhibits a substantially uniformspectral sensitivity characteristic to radiant flux having wavelengthsin the range 400 1,000 millimicrons, a current will be generated therebywhich current is proportional to the amount of incident radiant fluxnotwithstanding the spectral distribution of said flux. Photodiode 11additionally generates reverse saturation currents due to the thermallygenerated minority carriers inherent in the silicon conductor material.These reverse saturation currents, which are temperature dependent, arecommonly known as dark currents, and may exhibit a magnitude equal tothe magnitude of the current generated in response to incident low levelradiant flux.

The total current generated by photodiode 11 is supplied to resistancemeans 13 of the operational amplifier 12. The current is amplified bythe operational amplifier 12 in accordance with the relative values ofresistance means 13 and 14. A voltage is produced at the output terminalof operational amplifier 12 in response to the current supplied toresistance means 13. This voltage is applied to resistance means 17 ofoperational amplifier 16 whereby the applied voltage is selectivelyamplified. It is recognized that the applied voltage includes acomponent dependent upon the dark currents generated by photodiode 11.This component, if additionally amplified, results in erroneousindications of the intensity of the radiant flux incident on photodiode11. Accordingly the noninverting input terminal of operational amplifier16 is supplied with a current adapted to compensate for andcounterbalance the aforementioned dark currents. Thus the voltagepotential applied to terminals 31 and 32 induces a voltage potentialacross potentiometer 29 such that contact 30 supplies resistance means28 with a compensating current. The relative position of contact 30 maybe adjusted such that the effects of the dark currents on operationalamplifier 16 are substantially mitigated.

The voltage produced at the output terminal of operational amplifier 16is proportional to the voltage produced at the output terminal ofoperational amplifier 12 in accordance with the relative values ofresistance means 17 and a selected one of resistance means 26. Theindicating means 35 may provide a visual indication of the voltageproduced at the output terminal of operational amplifier 16. Adjustablecontact 34 of potentiometer 33 may be so adjusted as to supplyindicating means 35 with a portion of the voltage produced at the outputterminal of operational amplifier 16 so that indicating means 35provides a direct indication of the radiant flux incident on photodiode11. Hence potentiometer 33 serves to accuractely calibrate theindicating means 35. It should be readily apparent that if the magnitudeof the voltage produced at the output terminal of operational amplifier16 is not sufficient to result in a satisfactory indication byindicating means 35, the gain of the operational amplifier 16 may beselectively increased by properly positioning moveable armature 18.Alternatively if the magnitude of the voltage produced at the outputterminal of operational amplifier 16 is too great the gain of theoperational amplifier may be selectively decreased by a correspondingpositioning of moveable armature 18.

The aforementioned dark currents may be counterbalanced by the properdisposition of adjustable contact 30 of potentiometer 29. In the absenceof radiant flux incident on photodiode 11, dark currents supplied toresistance means 13 will result in a voltage pro duced at the outputterminal of operational amplifier 16 and a corresponding indicationthereof by indicating means 35. Consequently the adjustable contact 30of potentiometer 29 should be uniquely disposed to provide a voltage atthe non-inverting input terminal of operational amplifier 16 that isequal to the dark current voltage applied to the inverting inputterminal of operational amplifier 16. The applied voltages will canceleach other so that a null voltage of substantially zero magnitude willbe produced at the output terminal of operational amplifier 16, andindicating means 35 will correctly indicate that radiant flux ofsubstantially zero intensity is incident on photodiode 11. Furtheradjustment of contact 30 is not necessary since it is recognized thatthe drak currents generated by photodiode 11 are independent of incidentradiant flux.

Although operational amplifier means comprised of first and secondstages are illustrated in FIG. 1, it is obvious that the number ofstages utilized may be increased or decreased in accordance with thedegree of amplification desired while, preferably, maintaining a linearoperation of the illustrated apparatus. In addition, the relativeconnections between the first and sec- 0nd stages of the operationalamplifier means may be interchanged such that the operational amplifierexhibiting the selectively variable closed loop amplification factor maybe disposed in a preceding series relationship with respect to theoperational amplifier exhibiting the constant closed loop amplificationfactor. F urthermore one skilled in the art will recognize thatconventional frequency compensating circuits may be connected to each ofoperational amplifiers l2 and 16 to linearize the frequency responce toeach amplifier. Accordingly, the light meter in accordance with thepresent invention accurately measures illumination from light sourceshaving substantially constant outputs or from light sources havingfrequency variable outputs. The illustrated light meter may also beutilized to measure the illumination from a conventional flash lamp.

Referring now to FIG. 2 there is schematically illustrated anotherembodiment of the present invention comprising silicon photosensitivedevice 41, differential amplifier means 42, operational amplifiers 57and 59, and indicating means 70. The silicon photosensitive device 41 issimilar to the aforementioned device 11 and may comprise a photodiode ora phototransistor adapted to produce a current proportional to theintensity of radiant flux incident thereon from light source 10. Thecathode of photosensitive device 41 is supplied with a biasing voltage,and the anode thereof is coupled to a first input terminal ofdifferential amplifier means 42. A second input terminal of differentialamplifier means 42 is coupled to dark current compensating means furtherdescribed below. The differential amplifier means 42 is capable ofproviding an output voltage proportional to the difference between theinput voltages supplied thereto. Such differential amplifier means arewell-known in the art and need not be described in detail herein. Itshould be noted however that in the present application thereof it isdesirable to provide differential amplifier means having low noisecharacteristics and high input impedance. Accordingly, the differentialamplifier means 42 may include conventional field effect transistors,such as model 40673 manufactured by RCA, or may comprise an integratedcircuit FET differential amplifier.

Differential amplifier means 42 is coupled to operational amplifier 57which may be similar to aforedescribed operational amplifier 16. Hencethe operational amplifier 57 functions as an inverting amplifier havinga selectively variable closed loop amplification factor. The outputterminal of operational amplifier 57 is coupled to a moveable armature56 similar to the moveable armature l8 and adapted to selectively coupleone of stationary switch contacts 49, 51, 53, and 55 to the outputterminal of operational amplifier 57 in accordance with the relativedisposition of said moveable armature. Each of the stationary switchcontacts 49 55 is coupled to one terminal of an associated commonconnected resistance means 48, 50, 52 or 54. The common connectedterminals of the resistance means 48 54 are coupled to an input terminalof differential amplifier means 42. The closed loop amplification factorof operational amplifier 57 is determined by the relative magnitudes ofthe resistance of resistance means 48 54 and the relative disposition ofmoveable armature 56.

The output terminal of operational amplifier 57 is additionally coupledto operational amplifier 59 via resistance means 58. The operationalamplifier 59 is similar to aforedescribed operational amplifier 12 andis adapted to operate as a conventional inverting amplifier. The outputterminal of operational amplifier 59 is connected to an input terminalthereof via a feedback circuit comprised of fixed resistance means 60and adjustable resistance means 61. The adjustable resistance means 61may comprise a manually adjustable resistance means such as apotentiometer, a rheostat or the like, and serves as calibration means.Thus the adjustable resistance means 61 in conjunction with operationalamplifier 59, performs substantially the same function as potentiometer33 of FIG. 1. Another input terminal of operational amplifier 59 iscoupled to a reference potential such as ground potential via resistancemeans 62. The purpose of resistance means 62 is similar to that ofaforedescribed resistance means 15. Said other input terminal ofoperational amplifier 59 is additionally coupled to an offsetcompensating network via resistance means 63. The offset compensatingnetwork is comprised of potentiometer 64 having adjustable contact 65and voltage supply terminals 66 and 67. It

will be understood from the description set forth hereinafter that theoffset voltage inherent in operational amplifier 59 is minimized by theproper positioning of adjustable contact 65.

lndicating means 70 is similar to aforementioned in dicating means 35and is adapted to provide an indication of the amount of radiant fluxincident on photosensitive device 41 from light source 10. Accordinglyindicating means 70 may comprise a conventional voltmeter or the likeresponsive to a voltage supplied thereto. The indicating means 70 iscoupled to the output terminal of operational amplifier 59 via theadjustable contact 69 of potentiometer 68. The potentiometer is adaptedfor calibration of the indicating means 70 whereby an indication may bedirectly obtained in units of radiant flux per unit area such asmicrowatts per square centimeter. If desired, the potentiometer 68 maybe omitted and indicating means 70 may be connected directly to theoutput terminal of operational amplifier 59 whereby adjustableresistance means 61 provides suitable calibration means therefor.

In operation, the photosensitive device 41 supplies a first inputterminal of differential amplifier means 42 with a current proportionalto the intensity of the radiant flux thereon from light source 10. It isrecalled that the current produced by photosensitive device 41 includesa component consisting of dark currents. A second input terminal ofdifferential amplifier means 42 is supplied with a compensating currentequal in magnitude to the dark currents by resistor means 47. Thecompensating current is produced by inducing a voltage acrosspotentiometer 43 and by selectively positioning adjustable contact 44 soas to obtain a desired magnitude of compensating current. As isunderstood by those skilled in the art, the voltage produced bydifferential amplifier means 42 and supplied to the input terminals ofoperational amplifier 57 is proportional to the difference between thevoltages supplied to the differential amplifier means. Hence if thecompensating current is equal to the dark current, the differencetherebetween is zero, and the response of operational amplifier 57 tothe dark currents is minimized. It is noted that although the use ofdifferential amplifier means 42 for minimizing the dark currents ispreferable such differential amplifier means may be omitted and thecompensating current may be supplied directly to an input terminal ofoperational amplifier 57 by resistor means 47.

The magnitude of the voltage produced at the output terminal ofoperational amplifier 57 is dependent upon the selected closed loopamplification factor of said operational amplifier as determined by thedisposition of moveable armature 56 and the relative resistance ofresistance means 48 54. This has been discussed in detail with respectto the operational amplifier 16 of FIG. 1 and further explanationthereof is not necessary. It should, however, be noted that the producedvoltage is a function of the input current and said relative resistance.The amplified voltage produced at the output terminal of operationalamplifier 57 is further amplified by operational amplifier 59. It isunderstood that the gain of operational amplifier 59 is determined bythe sum of the resistances of resistance means 60 and 61 divided by theresistance of resistance means 58. This gain may be varied for thepurposes of calibrating the indicating means 70 by an adjustment ofadjustable resistance means 61. If such calibration is not necessary,adjustable resistance means 61 may be omitted.

The phenomenon of offset inherent in the operation of operationalamplifier 59 is minimized by providing a compensating voltage to aninput terminal thereofv The magnitude of this compensating voltage issubstantially equal to the inherent offset voltage and is derived byselectively positioning adjustable contact 65 of the potentiometer 64 towhich is applied a voltage potential. The magnitude of the currentassociated with said compensating voltage is effectively limited bycurrent limiting resistance means 63. Thus an offset voltage that may beassumed to be present at an inverting input terminal of operationalamplifier 59 is effectively counterbalanced by the compensating voltagesupplied to the noninverting input terminal thereof. Accordingly thevoltage produced at the output terminal of operational amplifier 59 islinearly proportional to the intensity of the luminous flux incidentonphotosensitive device 41, the dark currents and inherent offsetvoltages having been minimized.

The visual indication of the voltage produced at the output terminal ofoperational amplifier 59 is provided by indicating means 70. Said visualindication may be in units of light flux if adjustable resistance means61 and potentiometer 68 are properly adjusted. Although the combinationof adjustable resistance means 61 and potentiometer 68 admit of accuratecalibration of the indicating means 70 it is apparent that satisfactorycalibration may be obtained by utilizing only one of these components.

It should be clear from the foregoing description that the presentinvention is effective to provide an accurate, time stable, linearlyoperating light meter for measuring the illumination from a source oflight including a broad range of wavelengths. A plurality of lightmeters constructed as described hereinabove will exhibit substantiallyidentical operating characteristics.

While the invention has been particularly shown and described withreference to two embodiments thereof, it will be obvious to thoseskilled in the art that various changes and modifications in form anddetails may be made without departing from the spirit and scope of theinvention. For example, the operational amplifiers described above mayinclude field effect transistors to provide optimum noisecharacteristics. In addition the switch means comprised of a moveablearmature and stationary switch contacts may be replaced by other switchmeans such as relay switches, transistor switches or the like.Furthermore the compensating circuits utilized to counteract the effectsof dark currents and offset voltages may be replaced by otherconventional means adapted to satisfy the necessary objectives, andindicating means 35 and 70 may be calibrated to indicate the intensityof illumination from the source of light 10 in any desirable measuringunits such as lumens per square meter or other conventional units ofpower. The exemplary use of the light meter of the present invention isnot limited to those applications described hereinabove but encompassesall possible applications wherein a measure of illumination is desired.It is therefore intended that the appended claims be interpreted asincluding the foregoing and various other changes, modifications, andapplications.

What is claimed is:

l. Apparatusfor measuring the illumination from a source of light,comprising: i

a photodiode for producing a current proportional to the intensity ofillumination thereon; said photodiode being responsive to light havingwavelengths in the range 400 to 1000 millimicrons;

a differential amplifier coupled to said photodiode for receiving thecurrent therefrom; said differential amplifier being characterized by ahigh input impedance;

operational amplifier means coupled to said differential amplifier foramplifying said current; said operational amplifier means includingfirst and second series connected operational amplifiers, said firstoperational amplifier exhibiting a constant closed loop amplificationfactor and said second opera tional amplifier exhibiting a selectivelyvariable closed loop amplification factor;

adjustable means coupled to said differential amplifier for compensatingthe reverse saturation current thermally generated by said photodiode tomitigate the response of said operational amplifier means to saidreverse saturation current; and

means coupled to said operational amplifier means and responsive to saidamplified current for indicating the intensity of illumination on saidphotodiode.

2. The apparatus of claim 1 wherein said adjustable means comprises apotentiometer having an adjustable contact, said potentiometer having avoltage potential induced thereacross.

3. A light meter for determining the illumination on a plane from asource of light by measuring the amount of radiant flux received at saidplane, comprising a photodiode for generating a current proportional tothe amount of radiant flux incident thereon, said photodiode beingcharacterized by a substantially uniform spectral sensitivity to radiantflux exhibiting wavelengths in the range 400 to 1,000 millimicrons;

compensating means coupled to said photodiode for minimizing thermallygenerated reverse saturation currents inherent in the current generatedby said photodiode;

indicating means responsive to a voltage applied thereto for providingan indication of said applied voltage; 1

first operational amplifier means coupled to said compensating means,said first operational amplifier means including a vfeedback circuitcomprised of a plurality of resistance means connected in commonrelationship, each of said resistance means having a distinct value ofresistance, and switch means selectively connectable in series with oneof said common connected resistors, to thereby provide a selectivelyvariable closed loop amplification factor; and

second operational amplifier means intercoupling said first operationalamplifier means and said indicating means, said second operationalamplifier means having a continuously variable amplification factor toeffect calibration of said indicating means whereby said indication ofsaid applied voltage is in units of radiant flux per unit area.

4. A light meter in accordance with claim 3 wherein said compensatingmeans comprises:

a potentiometer having an adjustable contact, said potentiometer beingsupplied with a voltage potential; and

a differential amplifier having a first input terminal coupled to saidadjustable contact and a second input terminal coupled to saidphotodiode for supplying said first operational amplifier means with aninput voltage.

5. A light meter for measuring radiant flux of different wavelengthsreceived from a source of radiant energy, comprising:

a photodiode exhibiting a substantially uniform spectral sensitivitycharacteristic to said received radiant fiux for generating a currentproportional to the amount of received radiant flux;

a first operational amplifier arranged in closed loop configuration andcoupled to said photodiode, said first operational amplifier beingcharacterized by a fixed closed loop amplification factor;

a second operational amplifier arranged in closed loop configuration andcoupled to said first operational amplifier;

a plurality of resistance means selectively connectable in the closedloop configuration of said second operational amplifier such that saidsecond operational amplifier is characterized by a selectively variableclosed loop amplification factor;

adjustable dark current compensating means comprised of an adjustablepotentiometer coupled to an input of said second operational amplifierfor minimizing the response of said second operational amplifier toreverse saturation currents thermally generated by said photodiode; and

indicating means coupled to an output of said second operationalamplifier and responsive to a voltage produced by said secondoperational amplifier for providing an indication of the amount ofradiant flux received by said photodiode.

6. A light detector circuit having an improved dark current compensatingcircuit comprising, in combination:

a photodiode for producing a current proportional to the intensity ofluminous flux incident thereon;

1 l l 2 a potentiometer having an adjustable contact, said tionalamplifier means having a selectively variable potentiometer beingsupplied with a voltage potenclosed l lifi ti f t and indicating means,responsive to a voltage supa differential amplifier having a first inputterminal connected to said photodiode and a second input 5 terminalconnected to said adjustable contact;

operational amplifier means connected to an output said P terminal ofsaid differential amplifier, said operaplied by said operationalamplifier means to indicate the intensity of the luminous flux incidenton

1. Apparatus for measuring the illumination from a source of light,comprising: a photodiode for producing a current proportional to theintensity of illumination thereon; said photodiode being responsive tolight having wavelengths in the range 400 to 1000 millimicrons; adifferential amplifier coupled to said photodiode for receiving thecurrent therefrom; said differential amplifier being characterized by ahigh input impedance; operational amplifier means coupled to saiddifferential amplifier for amplifying said current; said operationalamplifier means including first and second series connected operationalamplifiers, said first operational amplifier exhibiting a constantclosed loop amplification factor and said second operational amplifierexhibiting a selectively variable closed loop amplification factor;adjustable means coupled to said differential amplifier for compensatingthe reverse saturation current thermally generated by said photodiode tomitigate the response of said operational amplifier means to saidreverse saturation current; and means coupled to said operationalamplifier means and responsive to said amplified current for indicatingthe intensity of illumination on said photodiode.
 2. The apparatus ofClaim 1 wherein said adjustable means comprises a potentiometer havingan adjustable contact, said potentiometer having a voltage potentialinduced thereacross.
 3. A light meter for determining the illuminationon a plane from a source of light by measuring the amount of radiantflux received at said plane, comprising : a photodiode for generating acurrent proportional to the amount of radiant flux incident thereon,said photodiode being characterized by a substantially uniform spectralsensitivity to radiant flux exhibiting wavelengths in the range 400 to1, 000 millimicrons; compensating means coupled to said photodiode forminimizing thermally generated reverse saturation currents inherent inthe current generated by said photodiode; indicating means responsive toa voltage applied thereto for providing an indication of said appliedvoltage; first operational amplifier means coupled to said compensatingmeans, said first operational amplifier means including a feedbackcircuit comprised of a plurality of resistance means connected in commonrelationship, each of said resistance means having a distinct value ofresistance, and switch means selectively connectable in series with oneof said common connected resistors, to thereby provide a selectivelyvariable closed loop amplification factor; and second operationalamplifier means intercoupling said first operational amplifier means andsaid indicating means, said second operational amplifier means having acontinuously variable amplification factor to effect calibration of saidindicating means whereby said indication of said applied voltage is inunits of radiant flux per unit area.
 4. A light meter in accordance withclaim 3 wherein said compensating means comprises: a potentiometerhaving an adjustable contact, said potentiometer being supplied with avoltage potential; and a differential amplifier having a first inputterminal coupled to said adjustable contact and a second input terminalcoupled to said photodiode for supplying said first operationalamplifier means with an input voltage.
 5. A light meter for measuringradiant flux of different wavelengths received from a source of radiantenergy, comprising: a photodiode exhibiting a substantially uniformspectral sensitivity characteristic to said received radiant flux forgenerating a current proportional to the amount of received radiantflux; a first operational amplifier arranged in closed loopconfiguration and coupled to said photodiode, said first operationalamplifier being characterized by a fixed closed loop amplificationfactor; a second operational amplifier arranged in closed loopconfiguration and coupled to said first operational amplifier; aplurality of resistance means selectively connectable in the closed loopconfiguration of said second operational amplifier such that said secondoperational amplifier is characterized by a selectively variable closedloop amplification factor; adjustable dark current compensating meanscomprised of an adjustable potentiometer coupled to an input of saidsecond operational amplifier for minimizing the response of said secondoperational amplifier to reverse saturation currents thermally generatedby said photodiode; and indicating means coupled to an output of saidsecond operational amplifier and responsive to a voltage produced bysaid second operational amplifier for providing an indication of theamount of radiant flux received by said photodiode.
 6. A light detectorcircuit having an improved dark current compensating circuit comprising,in combination: a photodiode for producing a current proportional to theintensity of luminous flux incident thereon; a potentiometer having anadjustable contact, said potentiometer being supplied with a voltagepotential; a differential amplifier having a first input terminalconnected to said photodiode and a second input terminal connected tosaid adjustable contact; operational amplifier means connected to anoutput terminal of said differential amplifier, said operationalamplifier means having a selectively variable closed loop amplificationfactor; and indicating means, responsive to a voltage supplied by saidoperational amplifier means to indicate the intensity of the luminousflux incident on said photodiode.